[keyring] keyring migration guide

[mattsb42-aws]

I'd like for us to have a migration guide for how to move from master key providers to keyrings. I'm not sure exactly what form this should take. The main things I want to make sure it does are:

• Show how to change from MKP to keyring. This is especially important for raw MKPs.
• Explain why you should change. ie, what do keyrings give you that MKPs do not?

[mattsb42-aws]

Thinking through this, I think that the best way to show how to migrate is by creating examples of common use-cases using both keyrings and master key providers.

[mattsb42-aws]

Apart from the examples, we decided that would fit best in the developer guide.